Degenerative feedback circuit



f July 14, 1942. R, B. ALBRIGHT DEGENERATIVE FEEDBACK CIRCU-IT v Filed July 24, 1927 Non@ f5r Y| fiEl?.

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'Patented July 14, 1942 UNITED s'rirresl PATENT .or-Fics DEGENERATIVE FEEDBACK CIRCUIT RobertBeniamin Albright, Philadelphia, Pa., as-

signor, by mesne assignments, to Philco Radio and Television Corporation, Philadelphia, Pa.,

, a corporation of Delaware Application July 24, 1931, serai No. 155,558

In Great Britain August 4, 1936 (ci. 1re- 171) imay be a triode, while the latter may be a 14 Claims.

This invention relates to audio amplifiers and the like of the type employing la feedback circuit and an automatic volume-expansion or volume-contraction circuit. By an automatic 4volume-expansion circuit is meant a circuit which causes the gain' of the ampliier to incre'ase with increasing signal strength, the rate of gain increase, however, occurring at such a slow rate as compared with the lowest frequency transl mitted by the amplifier, that substantially no modulation or distortion is introduced in the circuit. Such amplifiers are particularly suited for .use in phonographic reproducing apparatus, or in telephone or radio circuits, wherein at least part of the apparatus has a. limited volume range which is less thanthe volume range of the sound For example, the volume range employed in the playing of symphonic music by a concert orchestra may extend upward to 60 db. The volume range of a phono graph record however, is limited by physical and mechanical considerations to about 35 or 40 db. Tofimprove the realism of the reproduction, it is desirable to regain the lost volume range and this may be done by use of a volume-expansion circuit. A `volume-'contraction circuit operates in an opposite manner to reduce the volume range and may be used, for example, to compress the volume range employed'in the production of an orchestral selection so that it can be recorded on a circuit of this type in which the variable de-V generation is obtained in a simple and economical manner.

Other objects and features of the invention will be apparent from the following description and the accompanying drawing, in which Fig. 1 is a circuit diagram of a two-stage amplier including a. volume-expander feedback circuit;

Fig. 2 is a similar circuit diagram of an amplier employing a volume-contractor circuit; and

Figs. 3 and 4 are explanatory graphs of certain characteristics of the invention. f

Referring now to Fig. 1, there is shown a twostage amplifier employing a voltage amplier tube V1 and a power output tube Vz. The former pentode, and the output of the tube V1 may be transferred to the input circuit of V2 by conventional resistance-capacitance coupling as shown. The output of V2 may be supplied to a transformer T having a primary winding P-connected to the plate `or anode of the power tube V2 and a secondary winding S connected to some suitable signal utilization means, such as the loud speaker LS. Generally speaking, the loud speaker will have a very low input impedance, and consequently, the secondary S should have relatively few turns as compared with the primary P, so that the reflected impedance of' the power tube will match that of the loud speaker. It will be understood, of course, that other formsof power output stages may be used, and if desired, intermediate amplifier stages may be inserted between the first voltage amplifier and the output system.

In order to obtain degeneration, one side of the secondary S may be grounded and the other side may be connected to a voltage dividing network including resistances R1 and Rz, the two serially connected resistances being connected across the secondary S. The cathode of tube V1 may be connected to the junction of resistors R1 and Rz,

while the input signal supplied to V1 may be applied between the grid of tube V1 an'd ground. Thus, the actual voltage supplied to the input circuit of tube V1 will include the input voltage E and whatever voltage is formed across resistor R2. The signal E may be obtained from any suitable source such l as a microphone, a phonograph pick-up, or an audio detector of a radio receiver. The polarity of the windings ofthe transformer T should be so arranged that the voltage across R2 is of such polarity with respect to the signal E as to reduce the amplitude of the signal supplied to tube V1, that is the signal between the cathode and control grid of the tube. In this manner, a

degenerative feedback circuit is provided.

Considering the operation of this degenerative circuit, it will be apparent that the overall gain since the two volts output will cause a voltage of four-fths of a volt across resistor Rs, which will reduce the actual input voltage between cathode and grid to one-fth of a volt. The one-fifth volt amplified ten times, of course, produces the two volts output. The gain of the amplifier having been reduced five times, the amount of distortion, assuming the amplier to be not overloaded, will be reduced by a corresponding amount.

In accordance with the invention, ,the resistance R1 is caused to vary with the amplitude of the output signal. To this end, the character of resistor R1 is such that its resistance varies with the current owing through it. For example, R1 may be the resistance of a small incandescent lamp having a tungsten lament which has the property that its resistance increases as its temperature is raised by increasing the current flowing through it. The resistance of such a lament will increase by as much as times as the current through it is increased from some very small value (such that the iilament is not heated) to incandescence. As will be apparent from. Fig. 1, the amount of degeneration obtained will depend upon the proportion of the output Voltage fed back to the input circuit. For signals of small amplitude, the resistance of R1 will be low, since the lamp lament will not be appreciably heated, and eonsequently,'the maximum amount of degeneration will obtain. However, if the signal strength increases, the increased current ow through resistor R1 will increase its temperature, causing its resistance to increase; and the amount of.degeneration will decrease, since a smaller proportion of the output voltage will be fed back to theinput circuit.

In Fig. 3 there is shown a graph which illustrates the operation of the circuit. In this graph, the input voltage is taken as the abscissa and the output Voltage is taken as the ordinate. For any constant amount of degeneration, the ratio of input voltage to output voltage will be substantially linear and may be represented by a straight line passing through the origin of the coordinates. The slope of the line is a measure of the gain of the amplifier. As will be4V apparent, the maximum gain is obtained with lno degeneration. Line A represents this condition, While lines B, C, and D illustrate the ratio of output voltage to input voltage for increasing amounts of degeneration. The relative magnitudes of R1 and R2 may be so adjusted that when the output signal is of low amplitude the amount of degeneration would correspond to that represented by line D. As the output voltage increases, however. the resistance of R1 increases and the amount of degeneration decreases so that with increasing output voltage, the condition of the amplier is represented successively by curves C and B..

' Thus, the ratio of input and output voltages for the volume expansion circuit of Fig. 1, may be quired to change the temperature of the la-` ment. Fortunately, it has been found experimentally that the time required to change the temperature of the tungsten filament of a conventional lamp suited to this purpose, is sufliciently long so that no distortion of the signals,

having frequencies in the speech or audible range, takes place. 0n the other hand, the gain will change with suicient rapidity so that no noticeable delay will occur.

The output impedance of a standard moving coil loud speaker is of the order of magnitude of a few ohms. On the other hand, the impedances associated with the voltage amplifier tube V1 will be of the order of magnitude of tens or hundreds of thousands of ohms. Consequently, the resistances R1 and Rz may be of the order of magnitude of, for example, a thousand or iive thousand ohms. Hence, the voltage dividing network will not absorb any appreciable amount of energy from the output circuit and yet, with respect to the tube, the voltage divider will appear as a. signal source having a negligibly small internal impedance. Furthermore, the magnitude of R1 is such that ordinary tungsten lamps may be employed without requiring any special construction other than the judicious selection of the proper lamp from among those now available.

It is important to note that the resistors R1 and R2 provide an inherent biasing action whereby the bias of tube V1 is varied in accordance with the amplitude of the output signal. Due to the large impedance of said resistors compared to the impedance of the load, as mentioned above, the resistors are effectively in shunt with one another in the cathode connection to ground. Therefore, the D. C. voltage drop across said resistors serves to bias the tube V1. As the amplitude of the output signal increases, the signal current (A. C.) owing through the nonlinear resistor R1 increases, and hence the resistance of R1 increases. This increases the resultant resistance of the shunt-connected resistors, and since they are connected in the D. C. plate current path of tube V1, the D. C. voltage drop across the resistors increases, thereby applying an increased negative bias to the grid of V1. Thus, the bias of V1 is varied in accordance with the average amplitude of the output signal.

An. important advantage of the inherent biasing action above described is that it enables the amplifier V1 to handle input signals of greater magnitude without overloading. In other words, it increases the range of operation of the tube. 'Ihe system as a whole provides the desired volume expansion by means of the degenerative feedback connections, including the resistors R1 and R2, and at-the same time these elements serve additionally to provide the biasing action. The combined eiects contribute toward a greatly improved system.

While a tungsten lament lamp employed as the resistor R1 in Fig. 1 gives suicient volume expansion for most purposes, if it is desired to obtain a' wide range of volume expansion, this may be done employing a carbon filament lamp as the resistor R2, in addition to the tungsten lament lamp employed as the resistor R1. A carbon filament has the property that its resistance decreases with increased current ow through it, and therefore in this case, the voltage across Rz will be further decreased, as the output signal increases -in amplitude, due to the decreasing resistance of R2. 'I'he combined eiect of R1, and Rz will then be relatively greater volume expansion.

In Fig. 2 there is shown a similar circuit in which the feedback path is modified so that volume-contraction rather than volume-expansion obtains. Corresponding elements in this circuit are designated in the same manner as in Fig. 1.

. be seen then that this circuit operates as a vol- 'I'he operating curve of such a` ume contracter. circuit is shown at F in Fig. 3.

It will be apparent, of course, lthat various other forms of attenuating networks employing non-linear elements may be used. While the two forms shown in Figs. 1 and 2 are perhaps the simplest networks which may be obtained, more complicated networks may be employed if desired.

While the use of non-linear resistance elements, and particularly the use of lamp filaments as above described, is a simple and convenient method of varying the feed back signal in an amount determined by the amplitude of the signal in the signal channel, the invention contemplates the use of any.suitable method or means for thus varying the feed back signal. In other words, the invention contemplates the provision of a degenerative feed-back circuit in combination with any suitable means for varying the feed back signal as the signal inthe signal channel varies, to thus provide a; volume expansion or volume contracter circuit.-

In Fig. 4 there is shown a graph illustrating the sound quality which may be obtained, employing an amplifier constructed in accordance with the invention. In this graph, the power output is taken as the abscissa and may be plotted on a logarithmic scale, while the sound quality as determined by listening tests may be taken as the o rdinate. The quality of sound from an undegenerated conventional power amplifier becomes poorer at a rate substantially proportional to the log of the power output for low output levels, and the rate gradually increases as the output system is overloaded. Such a characteristic is illustrated by curve G. Where degeneration is employed, the quality'will be materially better up to the point at which overloading starts. At this point, however, the quality will become poorer very rapidly and will shortly become worse than that obtained without degenerating the amplifier. Such a characteristic curve is shown at H.

Following the practice of the present invention, the desirable characteristics of each of the curves G and H are obtained, which as far as is known is not characteristic of any other volume expander circuit. As will be apparent, for low and medium signal levels, there will be a considerable amount of degeneration in the device of Fig. 1 and hence, the quality characteristic for this amplifier will follow closely the curve H. However, as the output increases, the amount of degeneration decreases and hence, for large power outputs the quality curve will follow along the normal curve G for the amplier rather than along the'excessive distortion of the curve H in this region. Hence, a distortion curve for the amplier of Fig. 1 has the general shape shown by the curve J. It Vwill further be apparent that in the volume expander circuit of Fig. 1,

. the total possible gain of the amplier is available Cil on peak loads where ythe maximum possible gain is desired, since for these conditions -the amount of degeneration is reduced and appreciable degeneration is employed only where the output signal level is low and high gain isvnecessary.

While the volume contracter circuit of Fig. 2 does not obtain the same advantages from a point of view of distortion, this is not as important in this instance. since in general, such devices will not be employed to provide appreciable amounts of power and hence will have negligible distortion. In case such an amount of power is required a further power Iamplifier may be provided to meet these conditions. Furthermore, by. employing a volume contracter, the circuit may be designed so as to prevent any possible overloading of the latter device.

Itwillbe understood, of course, that theillustrated embodiments are for the purpose of disclosure only, and various modiications and furtherv embodiments may be resorted to in the practice of 'the invention.

I claim:

l. In a degenerative amplifier system, a vacuum tube amplifier having a pair of input elements and a pair of output'terminals, a plurality of resistors connected serially across said output terminals, at least one of said resistors having a non-linear, current-voltage characteristic, a conductive connection between one of said input elements and the junction of two of said seriallyconnected resistors, and means for connecting a source of input signals between the other of said v.input elements and one of said output terminals. 'said non-linear resistor being connected in the direct current plate-current path of at least one vacuum tube.

2. In a degenerative ampliiier system, a vacuum tube amplifier having a. pair of input elements and a pair of output terminals, a plurality of resistors connected serially across said output terminals, at least one of said resistors having aA non-linear current-voltage characteristic, a source of input signals, and means for connecting one of said resistors and said source serially between said input elements, whereby a portion of the totall voltage across said serially-connected resistors is included in the input circuit of the amplier to vary the gain thereof, said nonlinear resistor being connected in the direct current plate-current path of said amplifier.

3. In a degenerative amplifier system, a vacuum tube amplifier having a pair of input elements and a pair of output terminals, a source of input signals and a rst resistor serially connected between said input elements, there being a junction between said resistor and said source, a conductive connection between said junction and one of said output terminals, and a second resistor connected between the other of said output terminals and the input element to which said first resistor is connected, one of said resistors having a non-linear current-voltage characteristic both of saidresistors being connected in the direct current plate-current path of said amplier. f

4. In a degenerative amplifier system, a vacuum tube havinggat least a grid, an anode and current-voltage characteristic, a conductive connection between said cathode and the junction l of said seriallyconnected resistors, and means for connecting a source of input signals between said grid and one side of said load.

5. In a degenerative ampliersystem, a vacleast one of said resistors having a non-linear.

current-voltage characteristic, a conductive connection between said cathode and the junction of said serially-connected resistors, and means for connecting a source of input signals between said grid and one side of said load, said resistors and said load being included in both the grid-cathode and anode-cathode circuits of said vacuum tube.

6. In a degenerative amplifier system, a vacuum tube having at least a grid, an anode and a cathode, an output circuit for said amplifier including a step-down transformer, across whose secondary winding is connected a low impedance load, said load having a high potential terminal and a low potential terminal, a resistor connected between said cathode and the low potential terminal of said load, a source of anode voltage and an .anode load serially connected between said anode and said low potential terminal, a second resistor connected between said cathode and the high potential terminal of said load, at least one of said resistors having a non-linear currentvoltage characteristic, the series impedance of said resistors being high compared to the impedance of said load, said second resistor and said load, in series, being in shunt with said irst resistor with respect to the plate current in said vacuum tube, and a source of input signals connected between said grid and the low potential terminal of said load, the voltage drop across said shunt-connected elements being employed to supply bias voltage between said grid and said cathode.

'1. In a. degenerative ampliiler system, a vacuum tube having at least a grid, an anode and a cathode, an output circuit for said amplier in' cluding a step-down transformer, across whose secondary winding is connected a low impedance load, said load having a high potential' terminal and a low potential terminal, a serially-connected f pair of resistors shunted across said load, a second serially-connected pair of resistors connected between said low potential terminal and the junction of said iirst pair of resistors, said resistors having high impedances compared to the impedance of said load, and one of said resistors having a non-linear current-voltage characteristic, a source of anode voltage and an anode load serially connected between said low potential terminal and said anode, a conductive connection between said cathode and the junction of said second pair of resistors, and a source of input signals connected between said grid and the low potential terminal of said load.

8. In a degenerative amplier system, a vacuum tube having at least a grid, an anode and a cathode, an output circuit for said amplier including a step-down transformer across whose secondary winding is connected a low impedance load, a source of plate voltage connected between said anode and one side of said load, a pair of resistors serially connected across said load, the

impedance of said resistors being large compared to the vimpedance of said load, at least one of said resistors having a non-linear current-voltage characteristic, a 'conductive connection be -tioning simultaneously as a feedback network and as cathode bias resistors, whereby both feedback and bias are caused to vary in accordance with variations in amplitude of the signal across said load.

9. In a multi-stage cascade amplier system, input and output stages, each including a vacuum tube having at least anode, cathode, and grid elements, a coupling network connected between the anode-cathode circuit of said input stage and the grid-cathode circuit of said output stage, a load circuit connected to the anode and cathode elements of the vacuum tube in said output stage, a direct current path connected in shunt with said load circuit and comprising a pair of serially connected resistors at least one of which has a non-linear current-voltage characteristic, a direct current connection between the cathode of said input stage and the junction of said serially connected resistors, connections for including at least said non-linear resistor in the space-current path of the vacuum tube in said input stage, and a connection between the grid of said input stage and a point on one of said serially connected resistors other than said junction, whereby both the bias and the degree of feedback applied to said input stage are varied comprising at least two cascaded amplifier tubes,

a volume rang'e expander network comprising a negative audio feedback path connected between the output circuit of the second tube and the input circuit of the first tube, means operatively associated with said path for decreasing the feedback of audio voltage to said input circuit in response to increase of the audio amplitude, said means comprising a resistance device having a predetermined temperature coeicient, and said feedback path terminating in an impedance disposed in the space current path of the iirst tube.

l1. In an audio amplier system of the type comprising at least two cascaded amplier tubes, a volume range vexpander network comprising a negative audio feedback path connected between the output circuit of the second tube and the input circuit of the rst tube, means operatively associated with said path for decreasing the feedback of audio voltage to said input circuit in response to increase of the audio amplitude, said path terminating in an impedance disposed in the cathode circuit of the rst tube, said means comprising a temperature-responsive resistor device whose conductivity is directly dependent upon variations in said amplitude.

l2. In combination with at least two cascaded signal transmission tubes, a signal feedback path between the output and input of the second tube, and means, including an element in 'the space current path of said rst tube, for controlling lthe magnitude of signal feedback in response to the amplitude variation of the signal.

13. In an audio amplifier system, an amplier including an amplifier tube, a volume range variation network comprising a negative audio feedback path connected to the input circuit of A said tube, means operatively associated with said path for varying the feedback of audio voltage to said input circuit in response to variation of the audio amplitude, said means comprising a resistance device having a predetermined temperature coefcient, and said feedback path terminating in an impedance disposed in the space current path of said tube.

14. In an audio amplifier system. an amplifier including an amplier tube, a volume range variation network comprising a negative audio Cil ROBERT B. ALBRIGHYT., 

